Silicon carbide (SiC) has been used in applications requiring a hard, lightweight, temperature-and-wear-resistant material. SiC has good fracture strength, hardness, low theoretical density (ρ=3.21 g/cc) and thus relatively high strength/weight ratio. It is an attractive material for numerous applications. Conventionally produced SiC materials are manufactured using SiC powder processing and sintering. In this process, forming shaped products can be difficult and typically requires temperatures in excess of 2100° C. A number of chemical approaches based on polymer precursors have been developed for the synthesis of SiC (see for example, Laine et al., Chem. Mater. (1993), 5, 260; Richter et al., Applied Organometallic Chemistry (1997), 11, 71; Seyferth, Adv. Chem. Ser. (1995), 245, 131; and Birot et al. Chem. Rev. (1995), 95, 1443). Polymer precursors can offer some advantages over the conventional solid-state processing of SiC. However, some polymers lack the needed degree of processability or require difficult syntheses. The low char yields of most precursors lead to excessive shrinkage and cracking in the ceramic products and deterioration of mechanical properties. The ceramics are often rich in either Si or C, which again may lead to a degradation of the desired properties.
Polymethylsilane (PMS) syntheses give high-yield, near stoichiometric SiC ceramics upon pyrolysis. The syntheses involve producing pyrophoric polymers with the formula (CH3Si)x(CH3S1H)y, which must be further crosslinked by some mechanism (for example, borate (B[OSi(CH3)3)3]3) thermolysis) to give ceramics in high yield. Chain-terminating agents (such as (CH3)3SiCl) have been added to such systems, allowing ceramic yields of up to 64%. Polyvinylsilane has been added to PMS as a further pyrophoric ceramic precursor. Stabilization with 2,6 di-t-butyl-4-methylphenol (BHT) is generally required for all pyrophoric PMS syntheses. Syntheses of this type tend to be multistep and fairly complex.
Polysilynes were synthesized by Bianconi and Weidman in 1988 (Bianconi et al., J. Am. Chem. Soc. (1988), 110, 2342). The synthesis generally involves the reduction of alkyl- or arylsilicon trihalides with liquid NaK. High intensity ultrasound is used to ensure rapid and a more homogeneous reaction environment. These silicon-silicon bonded network polymers adopt a unique structure, in which each silicon bears one pendant group and is joined by three single bonds to three other silicon atoms, forming a continuous random network backbone. These silicon network polymers have a distinctive yellow color, very broad NMR resonances, and a broad and intense UV absorption band edge tailing into the visible. Recently Huang and Vermeulen have synthesized these network polymers electrochemically (Chem. Commun. (1998), 247). However, it has been reported that Wurtz coupling of methyltrichlorosilane yields a white intractable solid, unsuitable for processing into SiC (see Brough et al., J. Am. Chem. Soc. (1981), 103, 3049; West et al., J. Am. Chem. Soc. (1972), 94, 6110; Matyjaszewski et al., Polymer Bulletin (1989), 22, 253; Bianconi et al., Macromolecules (1989), 22, 1697; and Vermeulen et al., Polymer (2000), 41(2), 441.